Various strains of saprophytic soil bacteria are known to influence plant growth in different types of plants. For example, the inoculation of nonleguminous crops with selected strains of free-living, nitrogen-fixing species of Azotobacter and Azospirillum can cause significant increases in crop yield under field conditions. Kapulnik et al. (1981); Brown (1974). But bacteria of these genera are generally unable to compete adequately with native flora to assure multiplication. When used in seed inoculants, moreover, they are not "root colonizers," i.e., they are incapable of transferring in large numbers from seed to roots and, consequently, cannot keep pace with developing roots. See, e.g., Reynlers & Vlassak (1982). As a consequence, impractically large amounts of inoculum are required to obtain a meaningful effect on plant growth.
The mechanism(s) by which soil bacteria may influence plant growth has been the subject of extensive investigation. Research into the role of microbial iron transport agents (siderophores) in the root zones of plants (the "rhizosphere") indicates one mechanism by which some fluorescent pseudomonad species promote plant growth, namely, by antagonism (antibiosis, competition or exploitation) to deleterious indigenous microorganisms, resulting in their exclusion from roots. Kloepper et al (1980).
In this vein, it has been proposed that bacterial-mediated enhancement of plant growth generally involves interactions of the inoculated strain with rhizosphere microflora, possibly leading to the displacement of microorganisms detrimental to plant growth. For example, Canadian patent No. 1,172,585 discloses the use of particular strains of naturally-occurring pseudomonads to benefit plant growth in root crops by reducing the population of other indigenous root-zone microflora that adversely influence plant growth. Similarly, the results of one study indicated that growth-promotion in radish and potato by rhizobacteria did not occur under gnotobiotic conditions, when competition between other strains was not a factor, and hence, that rhizobacteria promote plant growth indirectly, by interaction of the rhizobacteria with native root microflora, rather than directly by microbial production of growth-promoting substances. Kloepper & Schroth (1981).
Another proposed mechanism for plant growth promotion by soil bacteria involves a direct stimulation of growth by bacterial elaboration of substances like nitrogen, plant hormones such as auxins and giberellins, and compounds that promote the mineralization of phosphates. But the hypothesis that elaboration of bacterial products is related to enhanced growth in plants has lacked definitive supporting data.
Thus, investigations of root-elongation promotion in grasses by an auxin-overproducer mutant of Azospirillum prompted the conclusion that observed levels of the bacterially-produced auxin bore no direct relation to the root elongation. A. Harari (1985). Using a petri plate bioassay for root elongation in wheat, Kapulnik et al (1985) found that seed inoculation with an A. brasilense strain resulted in root elongation in one bacterial concentration range but inhibition of root development in another, higher range. Kapulnik et al also reported that A. brasilense supernatants did not affect root length, a result militating against a substantial role for a bacterial product in promoting root elongation. A screening of rhizosphere bacterial metabolites for in situ effects on seedling root development likewise yielded mixed results, with the observed effects ranging from complete inhibition to unaffected development; notably, no growth stimulation per se was reported. Van De Geijn et al (1986).
Accordingly, there has been no clear indication heretofore that any soil bacteria might act directly to enhance root development, and certainly no showing of direct, bacterial-mediated stimulation of plant growth per se. Nevertheless, a bacterial strain capable of directly promoting plant growth, if one were isolated, could find immediate application, e.g., in soils where competition between introduced and native microflora did not result in a desired improvement in crop development. Such a direct-acting strain would be particularly useful under field conditions if it had the capacity to transfer from seed to developing roots, and to flourish in stable association with the root system of the maturing plant.